One of the principal objectives of aerospace research today is to develop a long range, hypersonic space vehicle which can journey from the earth to various points in outer space. Recently, vehicles of this type have originated from work under the National AeroSpace Plane (NASP) program. Some of the requirements of these vehicles include high density, low volume fuel; high speed travel (hypersonic range); relative light weight; and relatively large payloads. Fuels for these vehicles which have been considered include slush fuels, such as slush hydrogen.
The production of slush hydrogen (SH2) is currently accomplished by means of a "freeze/thaw" process. For the freeze/thaw process, the pressure in a container of liquid hydrogen (LH2) is progressively reduced by a vacuum pump. The resulting evaporation of hydrogen reduces the bulk temperature to the saturation temperature corresponding to the ullage pressure. The ullage pressure will be reduced to the triple-point (T.P.) pressure of 1.02 PSIA.
After the triple-point pressure is reached, further evaporation of hydrogen will convert part of the triple-point liquid to solid hydrogen. If the pressure is continuously maintained at the triple-point pressure, the frozen hydrogen will form as a solid sheet at the surface and ultimately suppress any further freezing action. Removing vapor at a low rate is also conducive to forming a solid sheet at the surface. Removing vapor at a high rate causes violent boiling and eruption of the bulk hydrogen, resulting in a large carry-over of liquid hydrogen droplets with the vapor flow.
Tests performed in the 1960's at the Bureau of Standards Cryogenics Laboratory showed that the optimum vapor withdrawal rate to minimize liquid hydrogen carry-over was approximately 160 CFM of vapor per square foot of liquid surface area. This flow rate was referenced to the vacuum pump inlet, and the inlet vapor had to be heated (approx. 500.degree. R, 40.degree. F.) because of vacuum pump mechanical limitations. The corresponding volumetric withdrawal rate at the liquid surface (approx. 24.8 R) was less by a factor of approximately 20, or 8 CFM per square foot of surface area.
To eliminate the formation of a solid sheet of hydrogen at the surface, the vacuum level is cycled above and below the triple-point pressure. During this portion of the cycle (about 1/2 the time) when the pressure is above the triple-point, the incoming heat (heat leaks and/or warm pressurant) causes part of the solid hydrogen at the surface to melt. Since solid hydrogen is more dense than liquid hydrogen, the former sinks in the remaining liquid. A mixer (propeller) in the bulk liquid assists in breaking up the solid hydrogen and in producing a more homogeneous mixture of the slush hydrogen.
The above-described process constitutes, for the most part, the currently-used freeze/thaw slush hydrogen production process. This process, as presently practiced, is a batch-type operation, and the slush production rate is relatively slow. Typically, slush Is produced only 50% of the time and a significant portion (about 30%) of just-formed slush is melted in the thaw cycle.
For example, the slush generator at the Slush Test Facility at Martin Marietta Corporation produces slush at the equivalent rate of approximately 9 gallons per minute (GPM). For a vehicle using slush hydrogen as its fuel (e.g., a NASP-type vehicle), the required slush generation rate during the density-maintenance operation is expected to be in the range of 1000 GPM to 4000 GPM, depending on the actual heat load to the propellant during the loading operation. It can be readily seen that a substantial improvement in slush production rate must be achieved if slush hydrogen is to be used to fuel a NASP-type vehicle.